In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon structure is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine patterns which are exposed on the photoresist material, thickness uniformity of the photoresist material is a significant factor in achieving desired critical dimensions. The photoresist material should be applied such that a uniform thickness is maintained in order to ensure uniformity and quality of the photoresist material layer. The photoresist material layer thickness typically is in the range of 0.1 to 3.0 microns. Good resist thickness control is highly desired, and typically variances in thickness should be less than .+-.10-20 .ANG. across the wafer. Very slight variations in the photoresist material thickness may greatly affect the end result after the photoresist material is exposed by radiation and the exposed portions removed.
Application of the resist onto the wafer is typically accomplished by using a spin coater. The spin coater is essentially a vacuum chuck rotated by a motor. The wafer is vacuum held onto the spin chuck. Typically, a nozzle supplies a predetermined amount of resist to a center area of the wafer. The wafer is then accelerated to and rotated at a certain speed, and centrifugal forces exerted on the resist cause the resist to disperse over the whole surface of the wafer. The resist thickness obtained from a spin coating process is dependent on the viscosity of the resist material, spin speed, the temperature of the resist and temperature of the wafer.
After the resist is spin coated and selectively irradiated to define a predetermined pattern, the irradiated or nonirradiated portions are removed by applying a developer. The developer is also spin coated onto the wafer by applying developer across the resist and then spin coating the developer until centrifugal forces disperse the developer over the coating of resist. However, the developer is not always uniformly dispersed on the wafer because of the difference of the surface area on the outer peripheral portion of the wafer in comparison to the center portion of the wafer. This may cause overdeveloping of the center portion of the resist on the wafer forming a developed resist layer with a thickness that is smaller at the center than at the outer peripheral surfaces. The result is the predetermined photoresist pattern cannot be formed.
The above stated problems even occur for nozzles that are designed to dispense developer more uniformly, such as a multiple tip dispensing nozzles. Typically, a multiple tip dispensing nozzle includes a chamber for receiving developer that distributes developer to a plurality of nozzles distributed over the length of the nozzle. Typically, the nozzle is connected to a moving mechanism by an arm. The arm moves from a rest position outside the wafer to an operating position over the center of the wafer. The nozzle is aligned such that different annular rings around the wafer receive developer, which is then spin coated onto the wafer. In some cases, the nozzle scan moves along a path as it dispenses developer to provide a more uniformly thick layer of developer on the photoresist material layer.
A prior art developer nozzle application system is illustrated in FIG. 1a. A multiple tip nozzle 10 is coupled to a pivotable arm 12 that pivots from a rest position 11 to an operating position 13. In the operating position 13, the multiple tip nozzle applies a developer on a resist layer 24 disposed on a wafer 22. The wafer 22 is vacuum held onto a rotating chuck 20 driven by a shaft coupled to a motor (not shown). The wafer 22 with its coating of resist 24 is rotated at a constant speed and the developer is applied through a plurality of tips 14 at different points along a radial line 30, so that developer is applied along different annular rings (not shown). The developer flows outward from each annular ring covering the entire top surface of the photoresist material layer 24. Although this type of application system improves the thickness uniformity of the coating of developer as opposed to moving a single nozzle over the center of the wafer, it does not always provide for the optimal thickness uniformity of the developer coating.
FIG. 1b illustrates an alternate prior art application system attempting to improve on the application system illustrated in FIG. 1a. The multiple tip nozzle 10 is coupled to the pivotable arm 12 on a slight angle. The nozzle moves horizontally from a rest position 15 to an operating position 17. In the operating position 17, the multiple tip nozzle begins to apply a developer on the photoresist material layer 24, and then scan moves the nozzle 10 from the center to the outer perimeter of the wafer 22, until the nozzle returns to the rest position 15. The wafer 22 with its coating of resist 24 is rotated at a constant speed and the developer is applied through a plurality of tips 14 along a translational path 32 The tips 14 are aligned on an angle at different perpendicular points along the translational path 32 to apply developer along different annular rings that are closer together than those that were discussed with reference to FIG. 1a. Although this improves the thickness uniformity of the developer coating, it does not necessarily provide for the optimal scanning path due to overlapping concentrations of developer being applied at the same general annular regions.
The resulting developer application systems illustrated in FIGS. 1a and 1b do not necessarily apply the optimal uniform thickness of developer, which may lead to a developed photoresist material layer that is not uniform and ultimately lead to impaired device performance. In view of the above, a system/method is needed, for dispensing an optimal uniformly thick layer of developer across a photoresist material layer formed on a wafer.